Xamarin iOS“演练-使用Elements API创建应用程序”崩溃

我试图遵循Xamarin的“演练-使用Elements API创建应用程序”，并且当我触摸“任务”时代码崩溃，该任务应该启动新屏幕以输入描述和日期选择器。单击NavigationItem.RightBarButtonItem时，将显示任务，但单击其中之一会导致应用程序在模拟器中关闭。

代码没有太多，而且我看不到哪里出了问题。

环境：
Visual Studio 2012> Xamarin构建主机> Mac OS 10.9.5

整个应用程序位于AppDelegate中：

using System;
using System.Collections.Generic;
using System.Linq;

using Foundation;
using UIKit;
using MonoTouch.Dialog;

namespace ElementsApiWalkthrough
{
    public class Task
    {
        public Task(){}

        public string Name { get; set; }
        public string Description { get; set; }
        public DateTime DueDate { get; set; }
    }

    // The UIApplicationDelegate for the application. This class is responsible for launching the 
    // User Interface of the application, as well as listening (and optionally responding) to 
    // application events from iOS.
    [Register("AppDelegate")]
    public partial class AppDelegate : UIApplicationDelegate
    {
        // class-level declarations
        UIWindow _window;
        RootElement _rootElement;
        DialogViewController _rootVC;
        UINavigationController _nav;
        UIBarButtonItem _addButton;
        int n;


        //
        // This method is invoked when the application has loaded and is ready to run. In this 
        // method you should instantiate the window, load the UI into it and then make the window
        // visible.
        //
        // You have 17 seconds to return from this method, or iOS will terminate your application.
        //
        public override bool FinishedLaunching(UIApplication app, NSDictionary options)
        {
            _window = new UIWindow(UIScreen.MainScreen.Bounds);

            _rootElement = new RootElement("To Do List") { new Section() };

            // code to create screens with MT.D will go here …

            _rootVC = new DialogViewController(_rootElement);
            _nav = new UINavigationController(_rootVC);
            _window.RootViewController = _nav;
            _window.MakeKeyAndVisible();

            _addButton = new UIBarButtonItem(UIBarButtonSystemItem.Add);
            _rootVC.NavigationItem.RightBarButtonItem = _addButton;

            _addButton.Clicked += (sender, e) =>
            {

                ++n;

                var task = new Task { Name = "task " + n, DueDate = DateTime.Now };

                var taskElement = new RootElement(task.Name){
                    new Section () {
                            new EntryElement (task.Name, 
                                    "Enter task description", task.Description)
                    },
                    new Section () {
                            new DateElement ("Due Date", task.DueDate)
                    }
                };
                _rootElement[0].Add(taskElement);
            };


            return true;
        }

    }
}